This invention is related to the production of electrospun fiber having very small coils which possess characteristics of structural chirality and can be used as negatively refracting structures, in photonics for the control of electromagnetic waves, or as mixtures of right and left handed coils for use as fibrous structures in medical applications.
It is known to produce nanofibers by using electrospinning techniques. These techniques, however, have been problematic because some spinnable fluids are very viscous and require higher forces than electric fields can supply before sparking occurs, i.e., there is a dielectric breakdown in the air. Likewise, these techniques have been problematic where higher temperatures are required because high temperatures increase the conductivity of structural parts and complicate the control of high electrical fields.
The technique of electrospinning or electrostatic spinning, of liquids and/or solutions capable of forming fibers, has been described in a number of patents as well as in the general literature. The process of electrospinning generally involves the creation of an electrical field at the surface of a liquid. The resulting electrical forces create a jet of liquid which carries electrical charge. Thus, the liquid jets may be attracted to other electrically charged objects at a suitable electrical potential. As the jet of liquid elongates and travels, it will harden and dry. The hardening and drying of the elongated jet of liquid may be caused by cooling of the liquid, i.e., where the liquid is normally a solid at room temperature; evaporation of a solvent, e.g., by dehydration, (physically induced hardening); or by a curing mechanism (chemically induced hardening). The produced fibers are collected on a suitably located, oppositely charged receiver and subsequently removed from it as needed, or directly applied to an oppositely charged generalized target area.
Fibers produced by such processes have been used in a wide variety of applications, such as in U.S. Pat. Nos. 4,043,331 and 4,878,908, where they useful in forming non-woven mats suitable for use in wound dressings. These U.S. patents make it clear that strong, non-woven mats can be made comprising a plurality of fibers of organic, namely polymeric, material produced by electrostatically spinning the fibers from a liquid consisting of the material or precursor. These fibers are collected on a suitably charged receiver and subsequently removed.
One of the major advantages of using electrospun fibers is that very thin fibers can be produced having diameters, usually on the order of about 50 nanometers to about 25 microns, and more preferably, on the order of about 10 nanometers to about 5 microns. These fibers can be collected and formed into non-woven mats of any desired shape and thickness. It will be appreciated that, because of the very small diameter of the fibers, a mat with very small interstices and high surface area per unit mass, two characteristics that are important in determining the porosity of the mat, can be produced.
Besides providing variability as to the diameter of the fibers or the shape, thickness, or porosity in any non-woven mat produced, the ability to electrospin the fibers also allows for variability in the composition of the fibers, their density of deposition, and their inherent strength. By varying the composition of the fibers being electrospun, it will be appreciated that fibers having different physical or chemical properties may be obtained. This can be accomplished either by spinning a liquid containing a plurality of components, each of which may contribute a desired characteristic to the finished product, or by simultaneously spinning, from multiple liquid sources, fibers of different compositions that are then simultaneously deposited to form a mat. The resulting mat, of course, would consist of intimately intermingled fibers of different material. Alternatively, it is possible to produce a mat having a plurality of layers of different fibers of different materials (or fibers of the same material but different characteristics, e.g. diameter), as by, for example, varying the type of fibers being deposited on the receiver over time.
As mentioned above, electrospinning involves the creation of a jet of fluid in an electrical field. The jet of fluid elongates and hardens or dries as it travels toward its target. The coils may be collected in various kinds of periodic and symmetric arrays, and random collections may also be useful. The rate of hardening or drying is also dependent on factors such as the path length of the jet of fluid. This, in turn, influences the physical characteristics of the non-woven article.
The characteristics of the coils and arrays of coils created by buckling of a fluid jet and by the electrically driven bending instability (Darrell H. Reneker, Alexander L. Yarin, Hao Fong and Sureeporn Koombhongse, “Bending instability of electrically charged liquid jets of polymer solutions in electrospinning”, Journal of Applied Physics, Volume 87, pages 4531 to 4547, May, 2000.)
Use of polymer coils, coated polymer coils of larger dimension is suggest by J. B. Pendry in Science, Volume 306, 19 Nov. 2004, pages 1353 to 1355, in a paper entitled “A Chiral Route to Negative Refraction”, which is incorporated by reference, and suggests that chiral resonances offer alternatives or advantages over negative refraction structures that are currently used. The terms chiral and chirality are usually used to describe an object which is non-superimposable on its mirror image. U.S. Pat. No. 7,106,918 teaches that structurally chiral materials can exhibit magneto-gyrotropy. The structural materials employed have at least one continuous structurally chiral material. Thus, these characteristics can lead to desirable properties and applications such as photonic structures or other applications.